Field of The Invention
The invention relates to a radiofrequency transmission device. The term “radiofrequency transmission device” is taken to mean any “hands free” access badge for accessing a motor vehicle, which communicates by radio waves with an on-board electronics unit in said vehicle, in order to unlock the doors of the vehicle without any need for the user to actuate the doors of said vehicle manually; the badge may take the form of either a card or a mobile phone.
Description of The Related Art
The term “radiofrequency transmission device” is also taken to mean a tire pressure sensor of a motor vehicle, either attached to the rim of a wheel of said vehicle, or placed in the tread of a tire of the vehicle. Said tire pressure sensor communicates by radio waves with a central on-board electronics unit in the vehicle in order to send pressure (and temperature) measurements made by said sensor in the tire to said unit, thus warning the user of any under-inflation of the tire.
Said radiofrequency transmission devices transmit radio waves at a predetermined nominal transmission frequency, referred to herein as the fundamental frequency fF.
A radiofrequency transmission device D of this type is shown in FIG. 1, and takes the form of an electronic circuit comprising, among other components:                a voltage supply source Vcc, which may be, for example, a battery, or the voltage drawn from the vehicle battery,        a transmission unit 10 for transmitting a voltage signal S, comprising an oscillator and a power amplifier,        a radiofrequency antenna A,        matching means M1 for matching the transmission frequency of the antenna A to the value of the predetermined fundamental frequency fF)        filtering means 30, generally comprising at least one band-pass filter.        
The transmission frequency matching means generally comprise:                a matching coil L, connected electrically in series with:                    a first matching unit 20, to match the frequency of the power amplifier of the transmission unit 10 to the fundamental frequency fF, supplied by the voltage source Vcc and connected, on the one hand, to said transmission unit 10, and, on the other hand, to the filtering means 30. Said first matching unit 20 generally comprises (see FIG. 2) at least a first matching capacitor C1 connected to the supply source Vcc and to the ground, a coil Lx connected to the supply source Vcc and to a junction point J, a second matching capacitor C2 connected to the junction point J and to the ground, and a third capacitor C3 connected to the junction point J and to the input of the filtering means 30,            and a second matching unit 40 for matching the transmission frequency of the antenna A, connected to the output of the filtering means 30, comprising one or more matching capacitors (not shown), and connected electrically to the radiofrequency antenna A.                        
Said radiofrequency transmission device D is known to those skilled in the art and will not be detailed further here.
The transmission unit 10 is supplied with voltage by the voltage supply source Vcc, and generates a pulsed voltage signal S (FIG. 1) whose predetermined fundamental frequency fF is substantially equal, because of the transmission frequency matching means M1, to the nominal transmission frequency of the antenna A of the radiofrequency transmission device D.
The generation of this pulsed voltage signal S is accompanied by the parallel generation of harmonic parasitic currents, that is to say periodic parasitic voltage signals whose frequencies are multiples of the predetermined fundamental frequency fF, that is to say frequencies equal to 2, 3, 4, and 5 times the predetermined fundamental frequency fF.
Said parasitic voltage signals are propagated throughout the electronic circuit of the radiofrequency transmission device D, and make the electronic circuit resonant. More precisely, the radiofrequency transmission device D then transmits radio waves at other undesired parasitic frequencies, in other words parasitic radio waves, in addition to the radio wave at the fundamental frequency fF. Said radio waves at the parasitic frequencies are propagated throughout the electronic circuit, said circuit generally consisting of a printed circuit, said waves being propagated through the copper tracks of the printed circuit, making the printed circuit resonant at the parasitic frequencies.
These parasitic radio waves may interfere with other on-board electronic equipment in the motor vehicle and may affect its operation.
The acceptable frequencies of on-board radiofrequency transmission devices D in motor vehicles are defined by the current national regulations in force in each country.
If said radiofrequency transmission devices D transmit radio waves at frequencies which are not legally authorized, the motor vehicle can no longer be officially approved.
It is therefore essential to prevent the transmission of these parasitic radio waves by the radiofrequency transmission device D.
For this purpose, according to the prior art, the filtering means 30 comprise a band-pass filter, generally in the form of an electronic circuit comprising capacitors and coils connected (not shown) in series and/or to the ground. This band-pass filter is matched in order to filter the voltage signals at parasitic frequencies and to allow only the voltage signal S at the predetermined fundamental frequency fF to pass to the radiofrequency antenna A.
The filtering provided in this way must be impedance matched before being connected to the antenna A. This impedance is adjusted on the basis of the fundamental frequency, by means of the second connected frequency matching unit 40.
However, since this band-pass filter is connected to the ground of the electronic circuit, the voltage signals at parasitic frequencies may be propagated in the ground plane of the radiofrequency transmission device D, which, in turn, then starts to radiate at said parasitic frequencies.
Furthermore, said low-pass filter requires a large number of electronic components, which is costly.